Fix RecursionError for duplicate files

[dvdl16]

Fixes #132

1. Guard associate_files() behind a flag so a save triggered from within association does not re-run association:

def validate(self):
    # guard against recursion: associate_files() can save another File, re-entering validate
    if not self.flags.associating_files:
        self.associate_files()
    ...

2. Set the flag before saving the duplicate in associate_files():

existing_file.flags.associating_files = True
existing_file.save()

3. Avoid duplicate file_association child rows - only append the association if the same link_doctype / link_name isn't already present:

already_linked = any(
   assoc.link_doctype == attached_to_doctype and assoc.link_name == attached_to_name
   for assoc in existing_file.file_association
)
if not already_linked:
   existing_file.append(
       "file_association",
       add_child_file_association(attached_to_doctype, attached_to_name),
   )

This breaks the recursion while still performing the intended one-pass association, and prevents redundant association rows from accumulating.

[dvdl16]

@agritheory Thank you for this app! As I'm not a collaborator, I don't think the failing actions have access to the necessary env vars. Let me know how you'd like me to proceed